Multi-hued Malachite

Mineral Matters #754

Malachite pseudomorphs after azurite from Whim Creek in Western Australia are famous. In fact, quite commonly, there is a double pseudomorph - chrysocolla after malachite after azurite.

This particular specimen of malachite after azurite is interesting for the range of greens, from a darker deep green through to a fibrous pale green. But what is the cause of this?

The answer most probably lies in the habit(s). The variation in the shades of green is most likely caused by a combination of physical and micro-chemical factors related to how the malachite crystals formed.

The darker green to almost black areas consist of larger, more densely packed, and well-aligned fibrous or acicular (needle-like) malachite crystals. When light hits these areas, it can penetrate deeper into the crystal structure before being reflected. More light is absorbed, and what is reflected is a deep, saturated green. In the image, these areas seem to be the core replacement of the original azurite blades, where the malachite fibres follow the original crystal's structure.

The lighter green areas are composed of much finer, more randomly oriented, and/or less densely packed malachite needles. When light hits this "felted" or "velvety" surface, it is scattered in all directions from the surfaces of countless tiny crystals. This high degree of light scattering mixes white light with the green reflection, resulting in a paler, less saturated, or even whitish-green appearance.

Think of a solid block of green glass versus that same glass crushed into a fine powder. The solid block is a deep, dark green, while the powder is a very pale green or almost white. The chemical composition is identical, but the physical form and its interaction with light are completely different.

And there are a few small pale hexagonal crystals to add a bit of extra interest. These could be either pyromorphite or mimetite. They “look” more like mimetite to me, but both species have been recorded here.

Below: Malachite after Azurite, Whim Creek Copper Mine, Whim Creek, Roebourne Shire, Western Australia. Width of view 6mm. Click on the image for a higher resolution version.